widgets.browsercolumn: Fixed segmentation fault due to 93601b17

https://github.com/hut/ranger/issues/45

hut:
  I've been experiencing irregular segfaults in ranger-master for which
  I can't figure out the reasons. Here is all the info I got:

  - It started around the time of commit 5417dda
  - I think it is a problem with curses' addstr/addnstr function
  - It happens randomly, there is no specific action that causes it
  - It happens more often with python3 than with python2
  - It's most likely somewhere in here: git diff master 5417dda5^
    ranger/gui/widgets/browsercolumn.py - but I don't see anything
    wrong.

hut:
  I nailed it down to 93601b1 and fixed it.

  For what it's worth, this is how to reproduce it: (tested with
  rxvt-unicode 9.15 and dwm but may work with any other tiling window
  manager):

  - Ensure that the setting display_tags_in_all_columns is set to True
    (this is the default)
  - Tag a file or directory with the "t" key
  - Navigate so that this tagged file is the LAST file you see of a
    column other the main column
  - Open a new window (resulting in rangers window getting smaller by at
    least one row)
  - Close a window (resulting in rangers window growing back to the
    original size)
  - Watch rangers brain being splattered all over the sand

  If you don't use a tiling window manager, you can do steps 1-3 and
  then resize rangers window very quickly.

  When resizing ranger manually, one of these messages is printed
  instead of "Segmentation Fault":

  - *** glibc detected *** python2.7: corrupted double-linked list: 0x0949cc98 ***
  - python2.7: malloc.c:3964: _int_free: Assertion `nextchunk->fd_nextsize->bk_nextsize == nextchunk' failed.

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